Interaction between integrin and extracellular matrix (ECM) mediates cell adhesion, and integrin-mediated cell adhesion regulates the activities and localizations of diverse signaling molecules, and consequently cell functions or behaviors (Thiery, J. P. Epithelial-mesenchymal transitions in tumour progression. Nat Rev Cancer, 2, pp. 442-54, 2002; Brakebusch, C. and Fassler, R., The integrin-actin connection, an eternal love affair, EMBO J, 22, pp. 2324-33, 2003). When integrins interact extracellularly to extracellular matrix (ECM) at focal adhesions, intracellular cytoplasmic tails of integrin subunits recruit diverse focal adhesion molecules, including adaptor proteins such as paxillin and p130Cas, and signaling molecules such as focal adhesion kinase (FAK) and c-Src (DeMali, K. A. et al., K. Integrin signaling to the actin cytoskeleton, Curr Opin Cell Biol., 15, pp. 572-82, 2003; Carragher, N. O. and Frame, M. C., Focal adhesion and actin dynamics: a place where kinases and proteases meet to promote invasion, Trends Cell Biol, 14, pp. 241-9, 2004). The recruitment of intracellular signaling molecules results in their activation, leading to reorganization of the actin cytoskeleton, and subsequently to changes in cell shape (Juliano, R. L. et al., Integrin regulation of cell signaling and motility, Biochem Soc Trans, 32, pp. 443-6, 2004). Since the integrins at the focal adhesions are linked to actin filaments through protein complexes (at their cytoplasmic tails), inefficient assembly of focal adhesion molecules or abnormal reorganization of actin filaments can cause cells to be round-shaped with concomitant loss of focal adhesions, leading to consequent detachment of cells from basement membrane (Hirohashi, S. and Kanai, Y., Cell adhesion system and human cancer morphogenesis, Cancer Sci, 94, pp. 575-81, 2003). In other words, since normal epithelial cells form a monolayer on the ECM-rich basement membrane, they can survive by effectively transducing extracellular signals from ECM. However, a loss of focal adhesion results in anoikis, leading to cell detachment from basement membrane, and consequently cell death.
Cancer cells due to multiple mutations and genomic instabilities may be allowed to disseminate from a primary tumor by abnormal alteration of cell-ECM interactions and cell-cell contacts. In the microenvironment of metastatic cancer cells, growth factors and cytokines secreted from cancer cells or neighboring fibroblasts, leukocytes, endothelial cells play an important role in cell contact, adhesion, migration, and invasion (Stamenkovic, I. Extracellular matrix remodelling: the role of matrix metalloproteinases, J Pathol, 200, pp. 448-64, 2003). Metastatic cancer cells traverse the basement membrane and stromal region to enter the circulatory systems (intravasation), and survive in the circulation. Then, they leave the circulation by extravasation. During this event, integrin-mediated adhesion of cancer cells to ECM may critically affect the metastatic potential of cells (Liotta, L. A., et al., Biochemical interactions of tumor cells with the basement membrane, Annu Rev Biochem, 55, pp. 1037-57, 1986). Among disseminated cancer cells, only cells survived anoikis may travel to distant sites via blood and lymphoid vessels, and eventually attach to a site and proliferate as metastatic tumors (Thiery, J. P., Epithelial-mesenchymal transitions in tumour progression, Nat Rev Cancer, 2, pp. 442-54, 2002).
Therefore, it would be useful to find reagent(s) to cause anoikis of tumorigenic cells without any effects on normal cells, when antitumorigenic or metastatic reagents are being screened.
TM4SF5 (or L6H) is a homolog of tumor-associated antigen L6, and forms a 4-transmembrane L6 superfamily with L6, IL-TMP, and L6D (Wright, M. D. et al., The L6 membrane proteins-a new four transmembrane superfamily, Protein Sci, 9, pp. 1594-600, 2000). TM4SF5 is highly expressed in pancreatic, gastric, colon, papilla vateri carcinoma and soft tissue sarcoma, and nonendocrine lung and ACTH (corticotropin)-negative bronchial carcinoid tumors (Pascual-Le Tallec, L. et al., Identification of genes associated with the corticotroph phenotype in bronchial carcinoid tumors, J Clin Endocrinol Metab, 87, pp. 5015-22, 2002). In the current study, we found that TM4SF5 is overexpressed in hepatocarcinoma tissues, and TM4SF5 causes actin reorganization and Epithelial-Mesenchymal Transition (EMT), leading to contact inhibition loss and oncogenic proliferation (Lee S-A et al., TM4SF5-mediated transmodulation between cytosolic p27Kip1 and Rho GTPases and epithelial-mesenchymal transition cause loss of contact inhibition, Cancer Cell, 2007). TM4SF5 overexpression in fibroblast led to actin reorganization and focal adhesion turnover (Lee, S. Y. et al., Focal adhesion and actin organization by a cross-talk of TM4SF5 with integrin α2 are regulated by serum treatment, Exp Cell Res, 312, pp. 2983-99, 2006). TM4SF (known as tetraspanin or tetraspan) proteins are a group of hydrophobic proteins of approximately 25-50 kDa with 4 transmembrane domains, and has two extracellular loops and two short cytoplasmic tails (Stipp, C. S. et al., Functional domains in tetraspanin proteins, Trends Biochem Sci, 28, pp. 106-12, 2003). TM4SFs form tetraspanin-web structures by forming complexes with cell adhesion molecules, such as integrins, to collaboratively perform their roles in cell adhesion and motility (Berditchevski, F Complexes of tetraspanins with integrins: more than meets the eye, J Cell Sci, 114, pp. 4143-51, 2001).